The present invention relates to the field of human impedance measurement devices.
It is known in the art to measure human impedance to monitor levels of intrathoracic fluids, such as blood. In particular, it is known to use an impedance monitor to measure human thoracic impedance, along with electrocardiogram (EKG) signals, as indicative of blood flow and heart performance characteristics, as described in U.S. Pat. No. 5,443,073(Wang et al.), the subject matter of which is incorporated by reference herein in its entirety. It is further known that certain medical conditions, such as congestive heart failure (CHF) or renal disease, correlate with the level and variation of the level of intrathoracic fluids.
Congestive heart failure results when the heart is unable to contract with sufficient vigor to meet the body's need for oxygen. Under such circumstances, to increase cardiac output, autoregulatory mechanisms allow the filling pressure in the ventricles to increase, thus elongating myocardial fibers at the start of systole and increasing the strength of contraction.
When the left and/or right filling pressures exceed approximately 15 mm Hg, blood components are forced out of the vasculature and into the interstitium, resulting in pulmonary edema (left heart failure) and/or peripheral edema and ascites (right heart failure). The end results are severe incapacitation and possibly death.
In a portion of the population with heart disease of many different etiologies—approximately 6,000,000 members of the U.S. population—the ability of the heart to meet the body's needs is marginal, resulting in chronic heart failure. For these patients, most of whom can be stabilized by medication and dietary restrictions and many of whom are quite elderly, minor variations in physical activity, emotional stress or non-compliance with diet or medication regimes can result in destabilization and episodes of acute heart failure requiring urgent hospitalization. Indeed, hospitalization for heart failure is the second most costly admitting diagnosis of the Medicare program.
There remains a need, however, for a practical and reliable method for monitoring the status of CHF patients outside of a hospital setting: with the goal of intervening before the onset of acute CHF. It would be most desirable to provide an easy-to-use and portable device for detecting increases in body water of patients with CHF before hospitalization is necessary and permitting adjustments in medication and/or diet in time to prevent an episode of acute heart failure. The present invention fulfills these needs and provides other related advantages.
The most reliable existing method to monitor CHF is by direct measurement of pulmonary artery and central venous pressures through catheters inserted into the bloodstream. This method, though highly accurate, is clearly impractical outside of a hospital setting. Other methods include observation of the arterial pressure pattern (invasively or noninvasively) during a Valsalva maneuver, measurement of flow though the mitral annulus and in the pulmonary veins using doppler echocardiography, observation of neck vein distension, measurement of ankle dimensions and careful tracking of body weight.
The first two methods, though reasonably accurate, require considerable equipment and trained personnel while the last three methods are quite unreliable for a variety of reasons.
Insertion of a cardiac catheter into the body may be hazardous. Its use can lead to death, which occurs in 1% of cases, and morbidity, which occurs in 33% of cases, as a result of infection and/or damage to the heart valves, cardiac arrhythmias, and pulmonary thromboembolism. Errors of technique, measurement, judgment and interpretation are common. It has been estimated that one-half million Swan-Ganz catheters used in the United States in 1986 resulted in the death of as many as 1000 or more patients. Furthermore, cardiac catheters cannot be kept in place for more than a few days owing to hazards from infection. They are also costly and labor intensive since catheterized patients require intensive care units which cost two to five times more than standard semi-private beds. In addition, health care workers face the risk of AIDS acquired virus and hepatitis virus as a result of exposure to blood of the infected patient during catheter introduction and subsequent maintenance. Moreover, cardiac catheters do not directly provide measurement of change in ventricular volume. While such measurements can be indirectly obtained in conjunction with injection of radiopaque dye and roentgenographic imaging, this technique is time-consuming and costly, and dangerous hypotension and bradycardia may be induced by the dye. Furthermore, the number of studies in a given patient is limited by the hazards of x-ray exposure and radiopaque dye injections.
Angiographic techniques provide the most widely accepted means for measuring ventricular volumes. They allow calculation of the extent and velocity of wall shortening and of regional abnormalities of wall motion. When they are combined with measurement of pressure, both ventricular compliance and afterload (i.e., the forces acting within the wall that oppose shortening) can be determined. When the results are expressed in units corrected for muscle length or circumferences of the ventricle, comparisons can be made between individuals with widely differing heart sizes.
Cineangiography provides a large number of sequential observations per unit of time, typically 30 to 60 frames per second. Although contrast material can be injected into the pulmonary artery and left atrium, the left ventricle is outlined more clearly when dye is directly injected into the ventricular cavity. Therefore, the latter approach is used in most patients, except in those with severe aortic regurgitation in whom the contrast material may be injected into the aorta, with the resultant reflux of contrast material outlining the left ventricular cavity.
Injection of a contrast agent does not produce hemodynamic changes (except for premature beats) until approximately the sixth beat after injection. The hyperosmolarity produced by the contrast agent increases the blood volume, which begins to raise preload and heart rate within 30 seconds of the injection, an effect that may persist for as long as two hours. Therefore, this technique cannot be utilized for repetitive measurements within a short time span. Further, contrast agents also depress contractility directly, though newer nonionic agents have been found useful for minimizing these adverse effects.
In calculating ventricular volumes or dimensions from angiograms, it is essential to take into account and apply appropriate correction factors for magnification as well as distortion produced by nonparallel x-ray beams. In order to apply these correction factors, care must be taken to determine accurately the tube-to-patient and tube-to-film distances. Correction is best accomplished by filming a calibrated grid at the position of the ventricle. Thus, angiographic methods do not have wide clinical application owing to their complexity, safety considerations, invasiveness, and side effects of the contrast agents.